PROJECT SUIVIMARY (See instructions): Role of key microbial populations of the gut microbiome in formulating host response as a result of 2,3,7,8- tetrahlorodibenzo-p-dioxin (TCDD) exposure is only beginning to be studied. Evidence suggests that gut microbiome is intricately linked to the host response and is also impacted directly and indirectly by exposure to environmental contaminants. In this project two such interactions are proposed to be studied using C57BL/6 germ free, mono-associated, cocktail associated and traditionally raised mouse models. The first is related to Treg/Th17 immune regulatory system and its dysbiosis in response to dioxin exposure and the second is related to choline-trimethylamine (TMA) pathway and its modulation. Dysbiosis of Treg/Th17 system has a broad range of health effects due to its central role in the immune regulatory system. It may lead to autoimmune diseases such as arthritis, inflammatory bowel disease, and skin cancer. Recently, specific members of the gut community, such as segmented filamentous bacteria (SFBs for which whole genome sequences are now available) and cluster IV and XlVa Clostridia, and Bacteroides fragilis have been implicated in regulating the Treg/Th17 system. The Treg/Th17 balance is also impacted by dioxin because AhR promotes the generation of Foxp3+ Treg and suppresses the differentiation to IL-17 producing ROR-yt Thi7. Thus specific populations of the gut microbiome, the Treg/Th17 balance, and TCDD via its AhR system are interconnected. Its mis-regulation may lead to disease outcomes. Project 4 establishes these specific and opposing mechanisms of regulation in germ-free mice using SFBs, 6. fragilis, and cluster IV and XlVa Clostridia and studies the role of these gut microbial populations in making the host less or more susceptible to health effects of dioxins. Evaluation of the transcriptome response of cluster IV and XlVa Clostridia and SFBs in response to TCDD, 2,3,4,7,9-PeCDF, and 2,3,7,8-TCDF exposure is also proposed. Project 4 also examines the effects of TCDD and related compounds on the metabolism of choline by the host and the gut microbiome, and the potential roles in the development of fatty liver and altering urinary trimethylamine/trimethylamine-N-oxide (TMA/TMAO) ratios. The effects are being studied in collaboration with Projects 1 (Kaminski/Kaplan), 3 (Zacharewski), 5 (Zylstra), and 6 (Boyd) and Research Support Cores A and B. The results are disseminated to communities and researchers through the Community Engagement Core and Research Translation Core. Student projects are designed to be incorporated in the Training Core.